Have you ever wondered why human beings look somewhat like chimpanzees and monkeys but quite different from bears? Or have you wondered how dolphins came to be such good swimmers? The science of evolution answers questions like these. Evolution means change in life through time.
Scientists think about evolution in two ways. First they think of all living things as being connected in a branchlike pattern, called the tree of life. Just as we can trace our own individual ancestry back through our parents, grandparents, and great-grandparents, we humans can trace our ancestry back to animals that were the ancestors of all primates (monkeys, apes, and humans), then to the ancestors of all mammals, to the ancestors of all animals, right back to the ancestors of all living things.
This branching pattern shows us how plants and animals are related to one another through their ancestors. It explains, for example, that gorillas are not our direct ancestors but that we share a common ancestor with them. More distantly, we also share common ancestors with dogs, frogs, jellyfish, and mushrooms.
The second way scientists look at evolution is through its usefulness in explaining why living things change over time. The most important reason is natural selection (explained later in this article). Natural selection helps scientists understand how and why living things evolve from one form to another.
Evidence of Evolution
Large changes in life-forms take place over thousands or millions of years. We cannot watch the changes take place and so cannot observe evolution directly. Instead, scientists look for various types of evidence that evolution has occurred.
The Fossil Record
Important evidence for evolution comes from fossils, which are the remains of ancient life. Fossils are often found in sedimentary rocks, which form when layers of loose material, such as sand or clay, are compressed over time. In all parts of the world, sedimentary rocks may contain the remains of plants or animals that lived when the rocks formed.
In the 1600s and 1700s, scientists began to realize that layers of sedimentary rock could be used as a kind of calendar for the Earth's history. Since the layers of rock built up over time, any lower layer must be older than those above it. Geologists were able to determine the ages of rock layers all over the world by comparing their positions in sequences of sedimentary rocks.
By about 1830, scientists had made an important observation: Different fossils are found in rocks of different ages. The oldest known rocks, which are about 3.8 billion years old, contain no fossils. Fossils of single-celled organisms, such as bacteria, are the only fossils found in rocks about 3.5 billion years old. Invertebrates with shells become common in rocks about 530 million years old. Eventually fish fossils appear, and in younger rocks are fossils of amphibians, then reptiles, and then mammals. Human fossils are found only in the youngest, and highest, rock layers. Thus the sequence of fossils shows that life on Earth has changed through time. It shows that evolution has taken place.
Fossils also show that certain groups of animals or plants have evolved from other groups. For example, fossils indicate that amphibians evolved from fish that were capable of breathing air and moving on land. The fossil Ichthyosteg is a transitional, or intermediate, form that demonstrates this. It had legs that were developed enough to carry it on land, but it also had many features of its fish ancestors, such as scales, a tail fin, and the same kind of teeth.
Transitional forms in other fossils show that reptiles evolved from a group of amphibians, that mammals evolved from a group of reptiles known as therapsids, and that birds probably evolved from dinosaurs. The birdlike fossil Archaeopteryx is a transitional form supporting this last idea. Like a bird, it had feathers and a wishbone. But it also had teeth, clawed "fingers" on each wing, a long bony tail, and other skeletal features that were nearly identical to those of small meat-eating dinosaurs.
Evidence from Modern Species
Even without a fossil record, there is other evidence that evolution has taken place.
Descent with Modification.
Different species often share similar features, which suggests that they inherited those features from a common ancestor. For example, the front limbs of all four-legged vertebrates have the same basic structure: one bone in the upper arm, two bones in the forearm, then the bones of the wrist and palm, then five fingers. This is as true for lizards, birds, bats, and whales as it is for human beings.
It is hard to imagine that this same bone arrangement is best for all the different actions of front limbs in these animals: running (lizards), supporting wings (birds and bats), swimming (whales), or grasping (humans). But it is easy to imagine that a common ancestor of these animals had this bone arrangement. The original structure of the front limbs was inherited with some changes by all the ancestor's descendants. English naturalist Charles Darwin (1809-82) called this descent with modification. It provides important evidence to scientists studying how animals may be related to one another.
Shared features between related animals sometimes appear only in the embryo form. For example, close study of human embryos shows that in early stages of development, human embryos have tails and gill arches, somewhat like those found in fish and chicken embryos. At birth, humans certainly do not have tails or gills. But the brief appearance of these structures during the embryo's development shows that they once existed in an ancient ancestor and were inherited by all its descendants.
Studies of certain molecules essential for life also provide strong evidence for evolution. Cytochrome C is a protein that helps organisms produce energy. Biologists have been able to determine the sequence of building blocks — called amino acids — that make up this protein. When they compare the 104-amino-acid sequence in human Cytochrome C to that in other species, they find that it is identical to the sequence in chimpanzees, 1 amino acid different from rhesus monkeys, 10 amino acids different from whales, and 35 amino acids different from wheat.
Similarities and differences of other proteins — including DNA, the genetic material itself — show that organisms share recent ancestry with some species and distant ancestry with others. Protein studies also give scientists information about how long ago two different organisms branched off from a common ancestor. For example, these studies support fossil evidence showing that humans and chimpanzees are descended from a primate ancestor that lived about 5 million years ago.
Some of the most unusual traits shared by different species are those called vestigial structures. These are the remains of structures that have been inherited from an ancestor but that have become useless. Vestigial structures can be understood in terms of descent with modification. For example, pythons have the remains of hind-leg bones, even though snakes do not have legs. However, the ancestor of snakes had legs. The python's useless hind-leg bones are vestigial structures.
The human appendix is also a vestigial structure. Useful for digestion in animals that eat mostly plants, it was important to our distant ancestors. In modern humans, however, it is of no use.
The way in which different species are distributed throughout the world provides additional evidence for evolution. For example, similar species are found together in certain areas. All types of kangaroos are found in Australia and nearby islands. This is because the ancestor of kangaroos lived in Australia, and it evolved there over time.
Also, specific plants and animals are not found in every place on Earth that could provide a suitable habitat. Tropical ocean islands are ideal habitats for frogs, but no frogs are found there. The reason for this is that the ancestor of frogs lived on the mainland, and no species were able to spread to distant ocean islands.
How Evolution Takes Place.
The foundation of modern evolutionary theory was laid by Charles Darwin. To explain evolution, Darwin needed to discover a reason why organisms would change over time. His explanation, called natural selection, is the most important part of the theory of evolution. (To a scientist, the word "theory" means a statement of what are held to be the general laws, or causes, of something known or observed. Theories are well-studied explanations that fit the available evidence.)
Natural selection is based on three basic facts about plants and animals in nature. The first is that individual organisms within a species vary slightly from one another and that these differences are inherited from parents to offspring. This is easy to see in human beings. We are all the same species, yet each person is unique. Children resemble their parents because they inherit traits from each parent.
The second fact is that in nature, plants and animals produce more offspring than can survive to adulthood. This causes a competition for limited resources such as food, which the organisms need to survive and reproduce.
Third, some individuals are better (or worse) competitors for these resources, based on their individual traits. Darwin concluded that organisms with traits that are helpful in a certain environment will, on average, survive and reproduce better than those with less favorable traits. The favorable traits, or variations, will be passed on to more offspring and will become more common in the overall population. This is natural selection.
A clear example of natural selection comes from a study of finches that live on the Galápagos Islands located in the Pacific Ocean — the same species seen by Darwin in the 1800s. Medium ground finches eat the seeds of plants growing on these small islands. Biologists studying the birds measured their beaks and found that the beaks varied in size, and — as expected — the beak size of offspring resembled that of their parents.
In 1977, no rain fell during the typical wet season on the Galápagos Islands. Plants did not grow or produce seed. Food became so scarce that many birds died. On one island the number of medium ground finches decreased from 1,200 to 180 that year.
Biologists observed that finches with smaller beaks died in greater numbers than large-beaked finches. They discovered that birds with larger beaks survived because they were able to eat the largest, hardest seeds that birds with smaller beaks could not crack open. In the struggle for limited food during a drought, large-beaked finches suddenly had an important advantage.
After the drought ended, biologists observed that the beaks of the next generation of finches were larger, on average, than before the drought. Natural selection, over the course of a single generation, had caused an increase in beak size.
Genetics and Inheritance. When Darwin developed his ideas about natural selection, he knew that offspring resembled their parents, but he did not understand how inheritance works. Scientists today know that a molecule called DNA contains the information that directs how an organism will develop. The information in DNA exists in sections called genes, which are made up of four building blocks called bases. The arrangement of genes and the arrangement of bases within each gene are the genetic code that makes each individual unique.
Each cell in an organism's body contains two sets of the genetic code, but only one set is found in a sperm or an egg. When a male and female reproduce, a sperm and an egg fuse to produce a single cell with two complete genetic codes, one set from the mother and one set from the father. From this cell an offspring develops. This is how we inherit traits from both parents.
Sometimes part of the genetic code changes accidentally. It may be miscopied, part of it may be left out, or an extra piece may be added. These are called mutations. Most mutations in genes are harmful, but not all. Some mutations produce slight variations in organisms without causing them harm, and, rarely, some mutations are beneficial.
In the example of medium ground finches, small mutations in the genes that control beak size produced, at some time in the past, some birds with slightly larger beaks. Larger beaks became an advantage during the drought, helping some birds survive.
In summary, mutations produce the variation on which evolution works. Without variation, there is no evolution, because there is no possibility of change.
Darwin was fascinated with the features of organisms that helped them fit so beautifully into their environment. Scientists call such features adaptations. For example, the long neck of the giraffe is an adaptation for eating leaves from the tops of trees. The bright color and fragrant scent of many flowers are adaptations for attracting insects that will pollinate the plant.
Adaptations result from the operation of natural selection. Ancestors of the giraffe did not have long necks. But those individuals that had slightly longer necks than others could find more food, simply because they could feed on leaves that others could not reach. Longer-necked giraffe ancestors therefore had more offspring than shorter-necked giraffes. The result, over many generations, is the giraffe we know today, the tallest land animal in the world.
Some fox species show clear adaptations to the special environments they inhabit. For example, the Arctic fox is superbly adapted to the tundra and polar ice of the far north. Its thick, white winter coat provides warmth and protective camouflage. Its hairy footpads help it walk easily in the snow. And its short well-furred ears minimize heat loss.
Adaptation to a similar environment can cause organisms to appear similar, even if they are not at all closely related. The body shapes of sharks, dolphins, and the extinct ichthyosaurs (prehistoric marine reptiles) are very much alike, even though one is a fish, another a mammal, and the third a reptile.
We know from fossil evidence that these species did not inherit this body shape from a common ancestor. Instead, it arose independently in each group. How? Natural selection worked to produce the same solution to the challenge of swimming fast in the sea. The shape is an adaptation to their underwater lifestyle. This is called convergent evolution. Each species converged, or moved together, toward a similar trait.
Speciation is the name for the process by which one species branches into two or more new species. Speciation most commonly occurs when a single species is divided into groups that live in separate geographic areas. This can occur when some individuals migrate to a new area or when the population is divided by some natural event, such as flooding, an earthquake, or a volcanic eruption.
When a species is geographically divided, individuals in one area no longer have contact with individuals in the other area. In particular, the two groups no longer reproduce with each other.
How does this separation produce new species? Natural selection does the work. For example, the environment in the two areas is unlikely to be identical. One area might be hotter or drier than the other. It might have more predators or different food resources. As the organisms struggle to survive, natural selection will favor those that are best suited to each environment. Over time, the populations living in the two areas will become increasingly different from one another. Eventually the two types will change, or evolve, into distinct species.
Once this happens, the process of speciation cannot be reversed. Even if the two groups came to occupy the same area again, they could no longer interbreed. In fact, this is how species are identified: They cannot breed with individuals outside of their species and still produce fertile offspring.
The speciation process described here is called allopatric speciation. It occurs because populations of organisms are living in other (allo-) places (-patric).
Rates of Evolutionary Change.
How fast does evolution take place? Sometimes it happens very quickly. In just a few decades, many species of insects have evolved the ability to survive insecticides, such as DDT. Viruses and bacteria also evolve quickly. Antibiotics, in general, are less effective today than when they were first widely used in the 1940s because some bacteria have evolved the ability to resist them. New virus species have recently evolved, including the AIDS virus and the virus that causes the canine disease called parvo.
The fossil record provides information about the rates of evolution throughout the Earth's history. It shows that in some kinds of organisms, small evolutionary changes occurred little by little over long periods of time, perhaps millions of years. This kind of slow change is called gradualism.
Other kinds of fossils show a different pattern of evolution. For example, clams and sea snails apparently evolved very little for a million years or more. Then, evolutionary change occurred quickly, perhaps during a few thousand years. This kind of evolution has been called punctuated equilibrium. Long periods of little change (equilibrium) are broken up, or punctuated, by shorter periods when new species form more rapidly. Scientists continue today to research which type of evolution is more common and in what kinds of fossils.
The fossil record includes a number of extinct species that belong to the same family as our species, Homo sapiens. The oldest members of the human family include several species of Australopithecus, a primate that lived in Africa between about 4 million and 1 million years ago. Australopithecus could walk upright, like a human; possessed a slightly larger brain than that of similarly sized apes; and showed several features that were intermediate between those of apes and modern humans.
Evolution and Religious Beliefs.
Some people do not accept evolution because it contradicts their religious beliefs. Most religions offer accounts of how God created the world and human beings. For example, Chapter 1 of the Book of Genesis in the Bible states that God created all living creatures in their present form during a six-day period. The view that this biblical account is literally true is called creationism. Creationists believe that life has not changed since God's creation of the world and that evolution's explanation of how life has changed through time must therefore be false.
In the early 1900's, creationists tried to keep ideas about evolution from being taught in public schools. Some states even passed laws making it illegal to teach evolution in public schools. In 1925, John T. Scopes, a Tennessee high school teacher, was convicted in a highly publicized trial of teaching evolution. The Scopes trial was often referred to as the monkey trial because, according to evolution, monkeys and humans share common ancestors. After this trial many years passed before schools began teaching evolution in science classes. Today creationists want their beliefs taught along with evolution.
Laws that would force creationism to be taught in public school science classes have been overthrown in United States courts. The courts ruled that creationism is a religious belief rather than a scientific theory. The United States Constitution does not allow the teaching of religion in public schools, in order to protect the religious freedom of all citizens.
However, many religious people accept evolution as the explanation for the history of life; some believe that God used evolution as a way of creating life.